Field of the invention
This invention relates to gas liquefaction systems, or “liquefiers”; and more particularly to a liquefier having an isolated liquefaction chamber adapted for dynamic pressure-control for achieving improved liquefaction efficiency.
Related Art
Gas liquefaction systems, also referred to as “liquefiers”, are well documented in the art and generally comprise a vacuum insulated container known as a Dewar, the Dewar being adapted to receive at least a portion of a cryocooler for liquefying gas, and further comprising a storage portion for storing an amount of liquefied gas therein.
FIG. 1 illustrates a liquefier comprising a Dewar 200 and a cryocooler 100 extending within a neck portion 206 of the Dewar. Within these systems, such a Dewar generally comprises an outer shell 202, an inner shell 201, and volume 203 therebetween being substantially evacuated of air to form a thermally insulated container. Optionally, a thermal shield 204 (shown in dashed lines), such as a foil or similar material, may be further disposed between the inner and outer shells of the Dewar. The Dewar further comprises a storage body portion 205 and the neck portion 206 extending therefrom. The Dewar is adapted to store a volume of liquefied cryogen within the storage body portion. A helium gas source 310 generally feeds an input gas line 211 for supply of the gas to be liquefied. A compressor 110 operates a first stage regenerator 101a for cooling a first stage 101b of the cryocooler, and up to several additional regenerators and cooling stages depending on the cryocooler design. The cryocooler 100 is illustrated as having three cooling stages comprising in addition to the first stage regenerator and first stage, a second stage regenerator 102a for cooling a second stage 102b, and a third stage regenerator 103a for cooling a third stage 103b. 
It is presently common for a cryocooler to comprise two or more cooling stages extending along a length of the cryocooler, such that a first stage thereof is adapted to pre-cool the gas and a subsequent stage is adapted to further cool the gas to a temperature sufficient for liquefaction. Moreover, each successive cooling stage typically comprises less surface area than the preceding stage, resulting in a cooling gradient along the several cryocooler stages.
Cryocoolers for use in such liquefiers and reliquefiers generally include a Gifford-McMahon (GM) type refrigerator or a pulse tube refrigerator; however these liquefiers may further include any type of refrigeration device for the purpose of cooling gases and condensing gas into a liquid phase. These liquefied gases are typically referred to as cryogenic liquids or cryogens.
Also documented in the art are “reliquefiers”, which generally comprise a liquefier that is adapted to circulate and re-liquefy gas within a closed or semi-closed system.
FIG. 2 illustrates such a reliquefier, which is substantially similar in design to the liquefier of FIG. 1. The reliquefier of FIG. 2 further comprises equipment 320 coupled in fluid communication with the Dewar for receiving an amount of liquid cryogen. Subsequent to using the liquid cryogen, evaporated gas is collected from the equipment and recycled back into the liquefier using a recirculator 315 such as a pump or similar device. It should be noted that the “equipment 320” may include one or more instruments, such as medical or scientific analytical instruments, among others, and is not limited to a single instrument of any design. Additionally, it should be noted that there exists a myriad of design variations which essentially recirculate collected gas back through a liquefier to form a closed or semi-closed system.
These liquefiers and reliquefiers, however, are limited with respect to liquefaction efficiency, or the amount of liquefied cryogen that can be generated using a given cryocooler over a period of time. There is a continued need for liquefiers having improved liquefaction efficiency.
Of importance to this invention are the thermodynamic properties associated with cryogen gases. These properties are generally illustrated through a phase diagram, such as illustrated in FIG. 3. In particular, the thermodynamic properties of helium gas are of great interest since liquefied helium is presently in high demand within a multitude of industries.
Now turning to FIG. 3, a phase diagram depicts a liquefaction curve for helium gas for various pressures (bar) and temperatures (Kelvin). The hexagonal close-packed (hcp) and body centered cubic (bcc) phases of the solid are shown for completeness. The liquefaction curve comprises a number of points at which helium gas transitions to liquid phase, the points collectively defining the liquefaction curve. A first liquefaction point (b) indicates a transition from gas-phase helium to a liquid-phase at a pressure of about 1 bar (near atmospheric pressure) which requires a temperature of about 4.22 K; this is known as the “boiling point” for helium-4, and hence point (b). A second liquefaction point (c) indicates the liquefaction of helium gas at a slightly increased pressure of about 2.27 bar which requires a temperature of about 5.20 K; this is known as the “critical point” for helium-4. In view of the liquefaction curve, it becomes recognizable that if a slightly higher pressure can be provided within the liquefaction chamber of the liquefier, liquefaction of helium gas can be achieved at slightly higher temperatures. Moreover, at these higher temperatures, most cryocoolers will be capable of increased cooling power. Thus, to take advantage of the higher cooling power of the cryocooler, one might develop a liquefier capable of liquefaction at pressures above 1 bar, and more preferably between 1 bar and 2.27 bar.
The advantages of liquefying a gas at pressures above 1.0 bar have been further described in WIPO/PCT Publication No. PCT/US2011/034842, by Rillo et al., filed May 2, 2011, and titled “GAS LIQUEFACTION SYSTEM AND METHOD”, the contents of which are hereby incorporated by reference. The Rillo system, however, merely describes embodiments wherein the cryocooler is positioned within the neck of a large Dewar such that the entire storage portion of the Dewar must be held at the elevated liquefaction pressure. This creates several serious problems: (i) Holding large cryogenic containers at high pressures is dangerous and further requires that the Dewar meet rigid safety requirements, thereby increasing the cost associated with the Dewar; (ii) before extracting the liquid cryogen, the Dewar pressure must be lowered to about 1.0 bar which results in the loss of a substantial amount of cryogen; and (iii) when lowering the pressure in the Dewar and removing the liquid cryogen from the Dewar, the system cannot simultaneously continue the liquefaction process at the optimum liquefaction pressure. To date, no instrument for liquefaction of gas has yet been developed that allows a gas to be liquefied at elevated pressures, stored at or near ambient pressures and further allows the user to extract the liquid cryogen from the Dewar while simultaneously continuing to liquefy gas at the optimal pressure. Such a system would also solve the problem of storing pressurized liquids and gasses at high pressures in large volume containers while realizing the benefits of pressurized liquefaction; i.e. increased efficiency. With increased efficiency, a smaller liquefier would be capable of replacing a larger liquefier while providing a similar liquefaction rate. Additionally, power would be conserved with the more efficient model.